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DHW set point for a tankless boiler?

Charles_8 Member Posts: 74
maybe one of these? They come in 1/2 inch.


In my 1-pipe steam system, the aquastat, which sets the high and low boiler temp, is in parallel with the thermostat. So if it's running frequently to keep the steam going (wall thermostat on), the aquastat high limit is effectively ignored, which is why you get such hot water. The aquastat only is effective at maintaining the boiler temp when the thermostat is NOT calling for heat.

The thermostatic tempering valve should help a lot. My system has one but it's corroded frozen so my HW temp varies a great deal, almost as though there weren't one. Haven't put the new one in yet. I'm sure the pros will add their advice shortly.



  • mikep7777
    mikep7777 Member Posts: 13
    Setting teh DHW temp on a Tankless system?

    Hi Guys,
    I know this is an elementary question, but I can't find a real answer on the web, hoping you could help me out.

    I'm a handyman, obviously not a heating pro. This weekend I visited a friend in CT who has a Burnham CB-3 boiler with a tankless DHW coil. It was cold outside - a high of 20 or so, low around 7, and the boiler seemed to be running pretty much continuously. The hot water was scaldingly hot - three other guests independently asked me to look at it, because they were concerned that someone would burn themselves badly.

    The high limit was at 180, the low was at 160 (probably because the Honeywell controller said on it that he low should be "20 degrees less than the high" instead of "at least 20 degrees less than the high". I turned the low limit down to 120, which seemed to make things better, but the water still seemed too hot.

    When the boiler is running pretty much constantly to provide heat, is there any way to reliably get DHW at a reasonable temperature, or will there always be at least some water in the DHW coil that gets overheated?

    I assume the correct way to fix the problem is to install a mixing valve. The hot water lines are 1/2", but the smallest mixing valve I could find was 3/4". Will a mixing valve work properly if its installed with 1/2 to 3/4 adapters?

    If I turn the boiler high limit down to 160, I assume this will affect the heat, but it would also improve the DHW situation - is this a reasonable short-term approach, or would the heating system be unable to keep up with the demand (given that it's running pretty much all of the time with the current settings)?

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